1. Field of the Invention
The present invention relates to a honeycomb structure. The present invention more particularly relates to a honeycomb structure which solves a problem of antinomy that it is difficult to satisfy both of a high temperature rise performance and a high thermal capacity at the same time and which is disposed upstream a filter for trapping particulate matters (PM) discharged from a diesel engine so that regeneration of the PM trapped by the filter can smoothly be completed and an exhaust gas can efficiently be purified.
2. Description of the Related Art
With tightening of regulation of an exhaust gas discharged from a diesel engine, various methods have been proposed in which a diesel particulate filter (DPF) is used in trapping particulate matters (PM) included in the exhaust gas from the diesel engine. In general, a method is adopted in which the DPF is coated with a catalyst that oxidizes the PM, and a honeycomb structure coated with the same catalyst is mounted upstream the DPF. In this honeycomb structure, the PM generated by depositing NO included in the exhaust gas as NO2 on the DPF are burnt. Alternatively, post injection is performed by controlling the engine. A non-burnt fuel is oxidized, an exhaust gas temperature is raised, and the PM deposited on the DPF are burnt and regenerated.
To smoothly burn and regenerate the PM deposited on the DPF, it is necessary to set a time when the catalyst with which the above honeycomb structure is coated reaches an activation temperature to be as long as possible. However, the diesel engine has a low exhaust temperature. Under a small load, the honeycomb structure does not reach the catalyst activation temperature. Even after an operation under a large load, during rapid transfer to the small load, the temperature of the honeycomb structure rapidly drops below the catalyst activation temperature in some case. There have been problems that a burning property of the PM is obstructed and that forced regeneration is not completed.
In view of the above problem, in general, countermeasures such as thinning of cell partition walls of the honeycomb structure and raising of porosity are performed to reduce a thermal capacity of the honeycomb structure and improve a temperature rise characteristic of a substrate. In consequence, the catalyst activation temperature is quickly reached.
When the thermal capacity of the substrate is reduced, the temperature rise characteristic of the substrate is improved, and the coated catalyst can quickly reach the catalyst activation temperature. Conversely, when the exhaust gas temperature drops, the temperature rapidly drops below the catalyst activation temperature. When the exhaust gas temperature drops, to inhibit the temperature drop of the substrate, the thermal capacity of the substrate is increased. In this case, the temperature rise characteristic deteriorates, and there is an antinomic relation between the thermal capacity and the temperature rise characteristic. That is, when the partition wall thickness and porosity of the substrate are simply changed to change the thermal capacity, it is difficult to lengthen a time when the catalyst with which the honeycomb structure is coated reaches the activation temperature.